1. Field of Invention
The present invention relates to a continuous heat treating furnace for a metal strip such as a continuous annealing furnace for annealing a continuously supplied steel strip or the like, and especially to a continuous heat treating furnace for a metal strip. The furnace is provided with a preheating section for preheating the metal strip to some temperature on an incoming side, and a heating section for treating the metal strip at a higher temperature.
In the annealing furnace exchanger for use in the invention, which anneals the metal strip, the temperature of the circulating gas to be blown over the surface of the metal strip in the preheating section is efficiently raised by re-circulating the heated exhaust gas from the preheating section.
2. Description of Related Art
A conventional continuous annealing furnace for continuously annealing a strip or a metal-strip continuous heat treating furnace is known wherein the furnace structure has a heating section for heating a metal strip to its transformation temperature A.sub.2 or higher. This heating device, constituted of multiple radiant tubes, is disposed around the continuously supplied strip. As the metal strip is supplied, if the necessary heat treating process is the annealing in a finishing process, the metal strip must be prevented from oxidizing. Since the heating temperature is high, oxygen components including CO.sub.2 and H.sub.2 O in the atmosphere of the furnace promote oxidization of the strip. Therefore, the annealing atmosphere of the strip needs to be at least a non-oxidizing atmosphere or a reduction atmosphere. In a burner which generates combustion exhaust gas including CO.sub.2 or H.sub.2 O, the in-furnace or atmospheric temperature cannot be directly raised.
To solve this problem, a high-temperature combustion exhaust gas or accordingly heated gas is supplied from the burner to the radiant tubes. Then, the strip can be heated with the radiant heat from outer walls of the radiant tubes. Consequently, by maintaining the in-furnace atmosphere as the non-oxidizing atmosphere or the reduction atmosphere, oxidization of the strip can be avoided as well as efficient heating of the supplied strip.
In a conventional continuous annealing furnace for annealing a metal strip or the like, by passing the heating-section exhaust gas or another combustion exhaust gas through the heat exchanger, heat is applied to the circulated gas. By blowing the gas over the metal strip passing through the preheating section, the temperature of the metal strip is raised.
Additional information pertaining to convective heat exchangers for recovering heat via tubes and regenerative burners is disclosed in Japanese published patent application 4-80969. A regenerative radiant tube burner is disclosed in Japanese laid open patent applications 6-257738 and 6-257724.
The foregoing related arts have problems. In an actual continuous annealing operation, to improve the production efficiency, the strip supply speed (plate passing speed) has a lower limitation. To improve equipment efficiency, the size of the heating section through which the strip passes should be as short as possible. To satisfy such a requirement, the in-furnace or radiant-tube temperature has to be set relatively higher than the desired ultimate strip temperature. Specifically, by raising the radiant-tube temperature, thereby increasing the difference between the in-furnace temperature and the strip temperature, the strip can be quickly heated to a predetermined higher temperature. However, by raising the radiant-tube temperature above the desired strip temperature, the radiant-tubes are subjected to additional thermal load and subsequent breakdown.
Specifically, thermal stress and high-temperature creep cause the radiant tubes to break. Their high-temperature life is deteriorated, and when the temperature of the radiant tubes is raised, the fuel consumption rate is increased, thereby disadvantageously increasing cost as well.
In the above first example, the high-temperature life of the radiant-tubes is shortened by several years. In the latter, the fuel consumption rate is directly reflected in increased cost. Therefore, economic constraints have focused improvements on decreasing the fuel consumption rate.
In an attempt to solve this problem, the combustion efficiency of the burner for heating the radiant tubes is raised. A sensible heat of combustion exhaust gas resulting from heating of the radiant tubes is recovered by a convective heat exchanger to a sensible heat of combustion air. Specifically, by increasing the temperature of the combustion air supplied to the burner, the combustion efficiency in the burner is enhanced.
Realizing the above solution, the operation line is provided with a preheating section for preheating the strip. In the preheating section, the sensible heat of the combustion exhaust gas from the burner is recovered as the sensible heat of a predetermined gas by a convective heat exchanger in the same manner as aforementioned. By blowing the heated gas directly onto the strip in the preheating section, the temperature of the strip can be directly increased.
However, in the aforementioned convective heat exchanger, combustion air, steam or another gas is passed through the tubes. Surrounding the tubes is the combustion exhaust gas. Therefore, via the tubes a sensible heat of the combustion exhaust gas is transmitted to the combustion air, steam or another gas for recovery. Hence, not only a sufficient difference in temperature between the combustion exhaust gas and the recovery gas must exist, but a large heat transmission area is also required. Even though large heat exchangers are available for recovering a sufficient amount of heat from the combustion exhaust gas, the installation space for these large exchangers is not available. Therefore, the heat recovery ratio is low.
Even if a sufficiently large heat transmission area is secured, it is difficult to heat the gas in the tubes in such a short time to a sufficiently high temperature. Thus, whether the combustion efficiency of the burner is enhanced with the convective heat exchanger, or the strip is preheated in the preheating section, the fuel consumption rate or the high-temperature life of the radiant tubes cannot be enhanced as expected.
To solve these problems, Japanese laid-open patent application 6-288519 discloses a continuous heat treating furnace in which continuous annealing is performed by using a regenerative burner device. In this reference, the regenerative burner device comprises of a pair of burners. One burner performs combustion, and a sensible heat of combustion exhaust gas is stored in the regenerator of the other regenerative burner. For example, when the temperature of the regenerator of the other regenerative burner reaches an upper-limit temperature and the combustion-heat reserve cycle reaches its limit, then that burner stops combustion, while the other regenerative burner performs combustion. Specifically, combustion air is passed through the regenerator of the other regenerative burner for combustion. In this case, the sensible heat of the combustion exhaust gas can be efficiently recovered as can that of the combustion air. Therefore, when the regenerative burner device is used as a burner in the continuous annealing furnace or another continuous heat treating furnace, the heat recovery efficiency can be enhanced. This hereby provides the expected reduction in fuel consumption.
In the regenerative burner device, each combustion burner needs to have a regenerator, which complicates the structure and increases the size of the device. In actual operation, however, the standard continuous annealing furnace or continuous heat treating furnace is provided with up to a hundred burners or heaters, while a larger furnace may contain hundreds of burners or heaters. If the burners or the heaters are replaced with regenerative heaters or burners, the structure is greatly complicated and enlarged. Not to mention the fact that it would be impossible to replace all the burners with regenerative burners or heaters because of the already restricted space. Additionally, control would become very laborious, which would disadvantageously complicate both maintenance and repair. Finally, it would be economically inferior to modify the existing equipment by replacing the usual burners with the regenerative heaters or burners.